Update txn field analysis to use comparison-return pattern

[S3v3ru5]

txn RekeyTo
global ZeroAddress
==
return

^Result of comparison is directly used as the return value. The comparison's result should be True for the successful execution of the contract implying that RekeyTo should have ZeroAddress.

This PR adds support to collect information from validations using the pattern Return(...).

[S3v3ru5]

Builds on #134